GB2519991A

GB2519991A - Apparatus and method

Info

Publication number: GB2519991A
Application number: GB1319453.5A
Authority: GB
Inventors: David Hall
Original assignee: DAVE HALL ENTPR Ltd
Current assignee: DAVE HALL ENTPR Ltd
Priority date: 2013-11-04
Filing date: 2013-11-04
Publication date: 2015-05-13
Anticipated expiration: 2033-11-04
Also published as: GB201319453D0; GB2519991B

Abstract

An apparatus 10 for inspecting the mounting of a component 12 on a substrate comprises a light source 16 for transmitting a beam of light beneath the component 12 from one side thereof, a detector 22 for detecting the amount of light passing under the component at a plurality of positions along an opposite side of the component, and processor means for comparing the amount of light detected at each position with an expected amount and determining whether the component is correctly mounted on the substrate in dependence on the comparison. Prisms 18, 20 or mirrors may be used to direct the light. The component may be a ball grid array (BGA) mounted with solder balls 14 and the inspection method may be used to test for dry joints or reflow inspection.

Description

APPARATUS AND METHOD

The present invention relates to a non-electrical contact method of inspection of the effectiveness of reflow solder processes for components on PCBs (printed circuit boards) and other substrates including stacked component assemblies. This includes the detection of shorts, missing solder balls, cold solder, misplaced solder balls and co-plainer compliance and correct height compliance on area gird array devices.

There are various techniques for soldering surface components. Solder paste can be applied to the substrate using a mask stencil or in the case of grid array components these can be supplied with a defined ball I column or bump of solder attached to each location of the contact array on the component (known as a grid array). The component is accurately placed onto the substrate I PCB then the composite is heated to melt (reflow) the solder on each joint.

The manufacturing facility will normally test the solder joints using X-ray inspection of product samples and I or at the end of the production line using functional test. Unfortunately a dry-joint' (where the solder has not made an alloyed joint may still make a mechanical contact sufficient to pass functional testing) may be prone to thermal movement and oxidization of the contact resulting in failure during the expected life of the product. A short between solder joints will not be detectable without the use of X-ray or electrical test. A missing solder ball maybe not be detected at electrical test if it is a ground or power connection.

The main aspect of the invention is to provide an improved technique for verification of the reflow solder process.

At present the detection of shorts and missing balls on area grid array devices is limited to X-Ray, manual microscope and electrical test on PCBs. As area grid array devices become smaller and more widespread the issues of shorts and missing balls will become more important and more of an issue. X-ray is not commonly used in fully automated lines due to speed of inspection, complexity and cost of the system. X-ray system, including 3D, are very good at finding shorts and missing balls but are normally used in batch test, during line set up or as a debug tools. Special microscopes are used to batch test, during line set up and as debug tools.

This is a manual operation and not suitable for automation. In-Circuit Test (ICT) and the flying probe testers rely on electrical access or a boundary scan path normally coupled to capacitive plates. None of these techniques will find most missing power or ground connections and access to the electrical signals is becoming much more difficult, therefore defect coverage is declining.

It is an aim of the present invention to provide an improved method or apparatus for inspecting FOB-mounted components and for detecting defects in the solder joints.

According to one aspect of the invention for which protection is sought there is provided an apparatus for inspecting a ROB-mounted component comprising transmitting a beam of light beneath the component from one side thereof, detecting the amount of light passing under the component from one side to the other, comparing the detected light with a reference pattern and determining correct soldering of the component in dependence thereon.

In an embodiment, the method comprises transmitting the beam of light substantially parallel to the POB or to a plane thereof. In an embodiment, the method comprises emitting a planar beam of light. In an embodiment, the method comprises scanning the light beam across from one edge of the component to the other.

According to another aspect of the invention for which protection is sought there is provided an apparatus for inspecting a FOB-mounted component comprising an emitter for emitting a beam of light underneath the component from one side thereof, a detector for detecting the amount of light passing under the component to the other side thereof, and a processor for comparing the detected light with a reference pattern and determining correct soldering of the component in dependence thereon.

In an embodiment, the emitter comprises one or more light emitting devices.

Other aspects and embodiments of the invention are set out in the claims appended hereto.

Some embodiments of the present invention enable FOB Reflow Inspection for shorts, missing solder balls, cold solder, misplaced solder balls and co-planar and correct height compliance on area grid array devices.

Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, except where such features are incompatible.

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows schematically an apparatus embodying one form of the invention; Figure 2 shows schematically an apparatus embodying another form of the invention; Figure 3 illustrates an example of an expected detection profile for light passing under a correctly mounted component; Figure 4 illustrates a first example of a detection profile for light passing under an incorrectly mounted component; and Figure 5 illustrates a second example of a detection profile for light passing under an incorrectly mounted component.

In embodiments of the invention, a light source is used to shine a beam of light under the mounted component from one side, and to detect it on the opposite side thereof. The inventor has recognized that the amount of light detected at positions corresponding to or aligned with the solder connections will be less than that at positions corresponding to or aligned with the spaces between the solder connections, such that if the component is correctly mounted on the PCB, an expected or predetermined pattern or profile of light will be detected on the opposite side of the component. Hence, by comparing the detected light profile with an expected profile, it can be determined whether the component is correctly or incorrectly mounted on the PCB.

Furthermore, the characteristics of the detected light profile can be used to estimate the nature of any incorrect mounting of the component. For example, if there is a short between one or more of the solder balls, then these will cause light to be blocked at the location of the short so that less light than expected will be detected at a position corresponding to or aligned with the location of the short. On the other hand, if solder balls are missing or if there are one or more open contacts, then more light than expected will be detected at a position corresponding to or aligned with the missing balls or open contacts. On the other hand, if the height of the solder balls is incorrect and/or the device is not co-planar with the RCB, indicating the presence of one or more shorts or one or more open contacts, then again the detection profile will differ from that expected.

This method could be used at sevelal different positions in the production line.

a. In-circuit Test (lOT), as illustrated in Fig 1; b. Functional Test; c. Standalone; d. Automatic Optical Inspection (AOl); e. Flying probe, as illustrated in Figure 2; f. Automatic 3D inspection (a3Di); g. Hand held debug; Figure 1 illustrates schematically an apparatus 10 for inspecting the mounting of a ball-grid-array (BGA)-type component 12 on a substrate such as a ROB (not shown). The component 12 includes a plurality of solder balls 14 which, when heated, soften in a process known as reflow to electrically connect the component to corresponding solder pads on the PCB.

The apparatus 10 comprises a first light source 16, in the form of an array of LEDs or other light emitting devices, configured to emit a wide beam of light downwardly towards the surface of the PCB. A first optical guide device, in the form of a mirror or prism 18, is disposed on or close to the surface of the ABC and adjacent to and substantially parallel with a first edge of the component 12. The first optical guide device 18 is configured to dived the beam of light through approximately 90° such that it is transmitted underneath the component 12 substantially plane parallel with the surface of the POB.

The apparatus 10 further comprises a second optical guide device 20 disposed on or close to the surface of the ABC and adjacent to and substantially parallel with a second edge of the component 12 opposite to the first edge. The second optical guide device 20 is also configured to divert the beam of light through approximately 900 such that it is incident on an optical detector in the form of a photoresistor array, photoelectric device or the like 22. The optical detector 22 is configured to detect the intensity or power of the light passing under the component 12 in a direction from the first edge to the second edge.

In the embodiment of Figure 2 the apparatus 10 comprises a second light source 24 and corresponding optical guide 26 configured to transmit a beam of light underneath the component from a third edge thereof orthogonal to the first and second edges. An optical detector and corresponding optical guide (not shown) are also provided adjacent to the edge of the component opposite to the third edge for detecting the intensity or power of the light passing under the component from the second light source.

Figure 3 illustrates an example of a trace generated by the or each optical detector 22 for a correctly mounted component 12. In this embodiment, the intensity of the detected light varies regularly along the width or length of the component in a manner corresponding to the positions of the solder balls 14. In particular, the strength of the detected light is at a minimum at positions along the edge of the component corresponding to the positions of the solder connections, which substantially block the light at those positions. On the other hand, the strength of the detected light is at a maximum at positions along the edge of the component corresponding to the spaces between the solder connections, which permit the light to pass through substantially unattenuated. Detecting a light profile of the kind shown in Figure 3 may be indicative of a correctly mounted component.

In Figure 4, the intensity of the detected light varies across the width of the component but in this case the magnitude increases from a value lower than expected at the left hand side of the graph to a value higher than expected at the right hand side of the graph. Such a detected profile may be indicative of a component that is tilted or inclined relative to the PCB, such that the left hand side is closer to the PCB and permits less light to be transmitted thereunder, and the right hand side is lifted away from the PCB and thus permits more light to be transmitted thereunder.

In Figure 5, there is a break in the detected light trace at a position corresponding to a space between the solder connections. Detecting a light profile of this kind may be indicative of a shorted connection between the solder balls which blocks the light from passing to the detector.

By comparing the detected light profile or trace with a theoretical optimum such as that shown in Figure 3, it can be deterniined whether the component is correctly mounted on the PCB. Such a comparison can be made by a control system such as microprocessor or the like.

It will be appreciated that inspection of the component may advantageously (but not essentially) be carried out in two substantially orthogonal directions so as to more accurately identify any faults in the solder connections. Inspection in only a single direction may not enable all faults to be revealed: for example a short between two solder balls in a particular row or column may not be detected in an inspection in which the light beam is parallel to that row or column.

The apparatus of Figure 2 is configured to perform such a two-dimensional inspection, having a first light source 16 arranged to direct light under the component in a longitudinal direction thereof, and a second light source 24 arranged to direct light under the component in a lateral direction thereof. Respective detectors detect the light transmitted under the component in each direction.

In the embodiment of Figure 1, on the other hand, which may be configured in the form of a so-called Flying Probe, the apparatus comprising the light source 16, the detector 22 and the two optical guides 18, 20 may be rotated after a first inspection of the component so as to perform a second inspection at right angles thereto. Alternatively, in some embodiments the component and substrate may be rotated relative to the apparatus to permit the second inspection in the orthogonal direction In both cases, the light profile detected by the or each detector can be overlaid or superposed such that the column and row at which any anomalous light reading is detected would indicate the X, Y coordinates of the fault.

It will be further understood that the optical guide devices are not essential and that the or each light source may be configured or arranged to emit the light beam directly beneath the component and parallel to the PCB and that the or each optical detector may be configured or arranged to directly detect the light passing under the component without redirection.

The light source(s) to be used can be selected as desired. In one embodiment, the or each light source comprises an array of LED5 or the like. The array may be configured such that all LED5 are activated simultaneously, so as to generate a relatively broad beam having a width comparable to that of the component under inspection. Alternatively, the array may be configured such that only a small number of LED5 are activated simultaneously, so as to generate a relatively narrow or pencil-like beam, having a width substantially less than that of the component under inspection. The latter arrangement may provide improved resolution compared to a wider beam, but may require the beam to be moved along the length or width of the component, either by sequential activation of adjacent LED5 or by physical moving of the array itself Control of the light source and detector, and movement of the apparatus where required, may be achieved by means of the above-mentioned control system and/or processor or by a separate system.

Each component or device to be inspected will require a clip or probe designed for that device size. These probes will be fitted into fixtures or be part of the automation on AOl and Flying probe. Each test platform will require an interface box (most likely USB) per tester; this will be a one off cost and will include the software license. Each UUT type will require a probe for every package type to be inspected. To summarize this method could be used across multiple test and inspection platforms as a low cost add-on to existing equipment.

Each device to be inspected will require a clip or probe designed for that device size. These probes will be fitted into fixtures or be part of the automation on a3Di, AOl and Flying probe.

A method of ensuring that small defects are detected will be required using a very sensitive, high resolution measurement system such as an integrating sphere. A narrow beam of light will be used as the light source with a method of scanning said beam along the length of two sides of the device being inspected. Known good devices will be used to generate a light power signature which will take the form of a complex waveform that will be compared to the scanned waveform of the devices being inspected. Waveform analysis will be performed in software to test for good or bad.

A mechanical design will be required that could include light guides, mirrors and fibre optic cables for both the light source and measurement system. The light source will shine parallel to the PCB and the underside of the component by the use of a small mirror.

The wavelength of the light source and detector will be selected so that visible light from other external sources such as sunlight and background lighting will have a minimum effect on the results. The light source may also be modulated also to reduce the effect of external light.